Despite recent advances in our understanding of pain mechanisms, there has been little-to-no overall improvement in the clinical management of postoperative pain. It is now recognized that effective postoperative pain management depends on key predictors, especially patient sex. Preclinical and clinical data indicate that, while postoperative pain levels show little-to-no sex-dependent dimorphisms, there are significant sex-based differences in analgesic efficacy, safety profile and abuse/addiction potential of current drugs used to treat postoperative pain. Hence, there is an urgent need to customize postoperative pain management schemes as based on sex-specific pain pharmacology. Our long-term goal is to define sex-dependent postoperative pain mechanisms, and utilize this knowledge to provide more effective postoperative pain management schemes. Our recent published and preliminary data show that a preclinical model of post-operative surgical incision pain leads to a sex-dependent up-regulation of prolactin (PRL) via extra-pituitary mechanisms at surgical sites and in the spinal cord. The PRL receptor (Prlr) is more responsive in female sensory neurons than in males. Moreover, administration of a Prlr antagonist at surgical sites, and especially in the spinal cord, suppresses postoperative hypersensitivity only in females and across all pain modalities. The objective of this proposal is to define peripheral and spinal mechanisms responsible for female-specific regulation of postoperative pain by the PRL system (i.e. PRL and Prlr). Our central hypothesis is that extra-pituitary PRL regulates postoperative pain in a female-specific manner via both peripheral and spinal mechanisms. The rationale is that understanding mechanisms contributing to the female-specific effects of PRL and Prlr on postoperative pain will 1) greatly expand knowledge of sex differences in pain mechanisms; and 2) provide translational potential for the development of novel therapeutic strategies specifically tailored for postoperative pain management in females. Our hypothesis is tested by interconnected yet independent aims. Aim 1 evaluates surgery-induced PRL and Prlr plasticity in peripheral terminals and innate immune cells at surgical sites, and central terminals and glia in spinal cord of female and males. Aim 2 defines the influences of sensory neurons, innate immune cells and glia in peripheral and spinal mechanisms of PRL- mediated regulation of postoperative hypersensitivity and ongoing pain in females and males. Aim 3 examines the regulation of nociceptor excitability by PRL in nave and operated females and males. The proposed study is innovative since if defines the sex-dependent role of extra-pituitary PRL in regulating postoperative pain, and pathways for specific regulation of postoperative pain in females. The proposed research is significant as it advances our knowledge of sex differences in postoperative pain mechanisms, and has substantial translational potential for new sex-based postoperative pain management strategies.